Olefin or styrene polymers are generally prepared by radical polymerization, ionic polymerization, or coordination polymerization using a Ziegler-Natta catalyst. Radical or ionic polymerization provides olefin or styrene polymers having mainly an atactic structure. Coordination polymerization using a Ziegler-Natta catalyst provides olefin or styrene polymers having mainly an isotactic structure.
The polymers are structurally divided into three groups such as atactic polymers, isotactic polymers, and syndiotactic polymers, depending on the position of benzene rings as a side chain in relation to a main chain of molecular structure of the polymers. An atactic structure means that configuration of side chains is irregular. An isotactic structure means that side chains are positioned at one side relative to a main chain. A syndiotactic structure means that side chains are alternatively arranged for a main chain.
The polymers having a syndiotactic structure have been only theoretically known, and practically not prepared until a metallocene catalyst was employed to the preparation methods.
Development of the metallocene catalysts was to provide a syndiotactic polystyrene having a stereoregularity or polyolefins having improved physical properties. The conventional metallocene catalysts have a structure which a Group IV transition metal compound of the Periodic Table of Elements is coordinated with ligands composed of one or two cycloalkanedienyl groups or their derivatives. The Group IV transition b metal of the Periodic Table of Elements contain titanium, zirconium, hafnium, etc. The cycloalkanedienyl groups include a cyclopentadienyl group, an indenyl group, a fluorenyl group, etc.
The metallocene catalysts are usually employed with a co-catalyst. The conventionally used Ziegler-Natta catalysts system is composed of a halogenated titanium compound as a main catalyst and alkylaluminium as a co-catalyst. Example of the halogenated titanium compound is titanium tetrachloride. Example of alkylaluminium is trimethylaluminium and triethylaluminium.
While, recently developed metallocene catalyst system is employed with alkylaluminoxane as a co-catalyst, which is capable of preparing polystyrenes having a stereoregularity or polyolefins having improved physical properties. Alkylaluminoxane is produced by reacting alkylaluminium with H2O. Especially, syndiotactic polystyrene has a structure which benzene rings as a side chain is alternatively positioned relative to a main chain of the polymer. The syndiotactic polystyrene has an excellent heat-resistance and physical properties since the polymer has about 270° C. of a melting point (Tm) due to stereoregularity comparing with the conventional amorphous atactic polystyrene, which is of interest.
European Patent Publication No. 210 615 A2 (1987) discloses a syndiotactic polystyrene having a stereoregularity. Also, the patent discloses cyclopentadienyltitanium trichloride and an alkyl-substituted cyclopentadienyltitanium trichloride such as pentamethylcyclopentadienyltitanium trichloride to prepare syndiotactic polystyrenes. It is known that the metallocene catalysts have a good activity, molecular weight distribution, and syndiotactic index.
U.S. Pat. No. 4,544,762 discloses a preparation method of aluminoxane which can be used as a component of catalysts in the preparation of highly active and homogeneous Ziegler-Natta catalysts.
U.S. Pat. No. 5,026,798 discloses a process for polymerizing α-olefins which utilize certain monocyclopentadienyl metal compounds of a Group IVb transition metal of the Periodic Table of Elements in an aluminoxane activated catalyst system to produce crystalline poly-α-olefins.
U.S. Pat. Nos. 08/844,109 and 08/844,110 disclose a new alkyl bridged binuclear metallocene catalyst (ABBM), silyl bridged binuclear metallocene catalyst (SBBM), and alkyl-silyl bridged binuclear metallocene catalyst (A-SBBM) for preparing a syndiotactic polystyrene having a high activity, a good stereoregularity, a high melting point, and a good molecular weight distribution.
Accordingly, the present inventors have developed new metallocene catalysts for use in preparation of olefin or styrene polymers and polymerization methods for effectively preparing such olefin or styrene polymers using the metallocene catalysts.